Comments on Inferences of Star Formation Histories and Birth Lines

Palla & Stahler have recently argued that star formation in Taurus and othernearby molecular clouds extends over a period of at least 10 Myr, implyingquasi-static cloud evolution and star formation. Their conclusions contradictother recent results indicating that molecular clouds are transient objects andstar formation proceeds rapidly. The Palla & Stahler picture implies that mostmolecular clouds should have extremely low rates of star formation, and that insuch inactive stages the stellar initial mass function should be stronglyskewed toward producing stars with masses $\gtrsim 1 \msun$; neither predictionis supported by observations. I show that the Palla & Stahler conclusions forTaurus depend almost entirely on a small number of stars with masses $\gtrsim 1\msun$; the lower-mass stars show no evidence for such an extended period ofstar formation. I further show that most of the stars apparently older than 10Myr in the direction of Taurus are probably foreground non-members. I alsopresent birthline calculations which support the idea that the ages of thestars with masses $\gtrsim 1 \msun$ have been systematically overestimatedbecause ``birthline'' age corrections have been underestimated; such birthlineswould eliminate the need to postulate skewed initial mass functions. Thesimplest and most robust explanation of current observations characterizing thevast majority of young stars in molecular clouds is that cloud and starformation is rapid and dynamic.Comment: 19 pages, 3 figures, to appear in Astrophysical Journa